A conventional chair adjustable device of a chair 60 is shown in FIG. 9 and generally includes a base 61 with a seat frame 62 and a backrest frame 63 respectively pivotably connected thereto. A pivot frame 64 extends from an underside of the seat frame 62 and the backrest frame 63 includes a hole 65 defined in a middle portion thereof and a pin 66 extends through the hole 65 and the pivot frame 64. The backrest is connected to the other end of the backrest frame 63. The backrest is pivoted, the backrest frame 63 is pivoted about a pivot end 67 of the base 61 and moves the pivot frame 64 which drives the seat frame 62 by the pin 66 and is rotated about the pivot end 68 of the base 61. U.S. Pat. No. 6,488,336 discloses a backrest adjustment device as shown in FIGS. 10 and 11, which includes a base 70 with a board 71 and an operation handle 80 is connected base 70. The handle 80 has a connection plate 81 and a pressing plate 82 to press on the pneumatic seat post 72. The board 71 includes a rail 731, a chamber 732 and guide frame 73 which includes a guide slot 733. A slidable frame 74 is slidably connected to the guide frame 73 and includes an inclined passage 741 and a lug 742. The slidable frame 74 is connected to the connection plate 81 of the handle 80 by a pivot 75 and a spring 76, so that the slidable frame 74 drives a positioning plate 77 which is movably engaged with the inclined passage 741. The positioning plate 77 protrudes from the guide frame 73 and can be engaged with the notches 91 in the seat frame 90 to position the seat frame 90.
For the device disclosed in FIG. 9, because the pivot frame 64 and the seat frame 62 are fixed with each other, the hole 65 must be large enough to allow the backrest frame 63 to be tilted. Nevertheless, when the backrest is pivoted back to its original position, the hole 65 is scrubbed by the pin 66 so that the inner diameter of the hole 65 becomes larger and larger which causes shaking of the backrest and makes the user feel uncomfortable. For the device disclosed in FIGS. 10 and 11, the mechanism is too complicated. When the handle 80 is pulled, the connection plate 81 and the pivot are moved to let the spring 76 move the slidable frame 74 which is moved along the rail 731 so that a component force coming from the inclined passage 741 moves the positioning plate 77. The movement is achieved by many indirect movements of different parts so that severe friction happens between these parts.
U.S. Pat. No. 6,378,943 discloses a chair tilt lock mechanism wherein the mechanism includes two springs 42, 44 and 92, 94, and 142, 144 to move slider 36 or the arm 136 so as to lock the tilt device. However, the two springs have to carefully set their modulus of elasticity and spring force, or the locking function cannot be precisely proceeded. Another U.S. Pat. No. 6,033,020 employs torsion spring 43 to drive the transmission part 42 so as to move the positioning plate 8 to engage with the notch 7. The positioning plate 8 is moved spirally by the torsion spring 43 so that the positioning plate 8 might not be functioned at the beginning of movement and noise due to impact between parts may happen.